The elimination or reduction of radar returns by the use on the target of materials that absorb electromagnetic energy at radar frequencies has been the subject of discussion at least since the time radar was widely used for military purposes in the second World War. That subject is discussed at pages 69 to 73, Volume 1 of the Radiation Laboratory Series, in the book titled Radar System Engineering, published in 1947 by McGraw-Hill Book Company. It is pointed out in that discussion that absorbers, in general, are of two types. The first type of absorber causes reflections occurring at the front surface of a layer of absorbing material to be canceled by destructive interference from waves that penetrate into the layer and are reflected back to the front surface. The second type of absorber is matched to the transmission medium, which usually is air, so that ideally there is no reflection of the incident electromagnetic wave energy at the front surface and all the wave energy that enters, ideally, is absorbed by the layer.
In recent times, instances of the use of absorbing materials on "spook" aircraft to make the aircraft invisible to radar have been reported. There is consequently a need for an improvement in radar to enable "spook" objects to be detected. This invention resides in a method of detecting such objects.
In the practice of the invention, short pulses of electromagnetic wave energy, preferably within the range of 1 nanosecond to 0.1 nanosecond pulse duration, having large relative bandwidth in the band from about 0.1 GHz to about 12.0 GHz are repetitively generated and directed by a scanning antenna or by a fixed directional antenna into the space to be monitored. Reflections of the pulsed wave energy from objects in the monitored space are then detected by a receiver. From tnose reflections, spook objects can be detected along with objects not having an absorptive coating. By operating in accordance with the invention, a coating of absorbing material, to be effective, must be so thick that it becomes impractical to provide a vehicle with a coating of the requisite thickness .